Output file has an extra \n character between each line on windows

[nocalla]

This looks like a manifestation of the handling of Windows files outlined here,

Be careful using codecs.open() to read files on Windows. The docs say: """Note Files are always opened in binary mode, even if no binary mode was specified. This is done to avoid data loss due to encodings using 8-bit values. This means that no automatic conversion of '\n' is done on reading and writing.""" This means that your lines will end in \r\n and you will need/want to strip those off.